A contour on an earthworks drawing, the centreline of a bend, the edge of a landscaped pond, a flexible duct. All four are smooth curves that follow no particular arc, and building them out of joined arcs is both slow and visibly kinked.
The spline command in AutoCAD draws exactly that kind of line. But it has two methods, and the same string of points gives two curves that differ by a factor of two. Not knowing which one you are in means drawing something you did not intend. For the full drawing toolkit see our AutoCAD for Construction course.
Where to start it
Read from the acad.cuix interface file and the acad.pgp file of the 2027 release.
| Way in | What to use |
|---|---|
| Full command | SPLINE |
| Alias | SPL |
| Ribbon | Home → Draw → Spline CV |
| Classic menu | Draw → Spline → Control Vertices |
| Toolbar | Draw → Spline |
Notice the ribbon button reads Spline CV. Pressing it goes straight into the CV method, while typing SPL picks up whichever method was used last. That is the real reason behind "yesterday it drew one shape, today it draws another".
The steps on the command line
Copied verbatim from AutoCAD 2027, in the default method.
Command: SPLINE
Current settings: Method=Fit Knots=Chord
Specify first point or [Method/Knots/Object]:
Enter next point or [start Tangency/toLerance]:
Enter next point or [end Tangency/toLerance/Undo]:
Enter next point or [end Tangency/toLerance/Undo/Close]:| Step | What it asks | What you do |
|---|---|---|
| 1 | Specify first point or [Method/Knots/Object]: | Pick the first point |
| 2 | Enter next point or [start Tangency/toLerance]: | Pick the second |
| 3 | Enter next point or [end Tangency/toLerance/Undo/Close]: | Keep picking, Enter to finish |
The first line matters most and nearly everyone skips it: Current settings: Method=Fit Knots=Chord. The program prints the method in force right before it asks for a point. One glance tells you what sort of curve you are about to draw.
Switching to the other method means typing m then cv, and the prompts change with it:
Specify first point or [Method/Knots/Object]: m
Enter spline creation method [Fit/CV] <Fit>: cv
Current settings: Method=CV Degree=3
Specify first point or [Method/Degree/Object]:
Enter next point:
Enter next point or [Undo]:
Enter next point or [Close/Undo]:The option list is quite different: the Fit method offers Knots, the CV method offers Degree. The square brackets alone tell you where you are.
How the two methods differ
This is the heart of it, and measuring is the clearest way to see it.
Take four fixed points with 700 between the lowest and the highest. Draw three things through those same four points and measure the height of the bounding box.
| Drawn with | Height of the box |
|---|---|
| A polyline through the four points | 700 |
| The fit method | 1,011.46 |
| Control vertices | 510 |
Three numbers say it all.
The fit method passes through the points you picked, but to stay smooth it has to swing outside the range they cover — 311.46 beyond, more than 44% of the original spread.
The CV method touches none of the points. What you pick is a control frame, and the curve stays neatly inside it: 510 against 700.
Which method for which job
The two figures above settle the choice.
Where the line must pass through the points, use the fit method: joining survey points, drawing a contour through known levels, setting out a centreline through fixed marks. Remember the curve will swing outside them, so check the clearances if anything sits close on either side.
Where only the shape matters, use control vertices: the edge of a pond, the outline of planting, a decorative line. Dragging one frame point reshapes the whole run smoothly, and it is far easier to adjust.
The figures read off the objects themselves:
| Fit method | CV method | |
|---|---|---|
| Points passed through | 4 | 0 |
| Frame points | 6 | 4 |
| Degree of the curve | 3 | 3 |
The fit method quietly adds two frame points you never picked, and that is precisely how it forces the curve through all four.
Autodesk's SPLINE page describes the same two routes: "Splines are defined either with fit points, or with control vertices."
The Knots option changes how far it swings
The fit method has one more setting few people open: Knots. The opening line prints its current value, Knots=Chord by default.
It decides how the program divides the curve between the points. Autodesk describes three ways.
| Method | What Autodesk says |
|---|---|
Chord | "Spaces the knots connecting each component curve to be proportional to the distances between each associated pair of fit points." |
Square root | "Spaces the knots connecting each component curve to be proportional to the square root of the distance between each associated pair of fit points." |
Uniform | "Spaces the knots of each component curve to be equal, regardless of the spacing of the fit points." |
In short: Chord divides by the real distances between points, Square root by their square roots so the curve comes out gentler, and Uniform divides equally whether the points are close together or far apart. Where survey points are unevenly spaced and the curve swings too far, Square root usually gives a more sensible shape.
Turning a spline into a polyline
Plenty of analysis packages, most CNC cutters and some estimating software cannot read splines. Then the curve has to become a polyline.
Type PEDIT, pick the spline, and this comes back verbatim:
Command: PEDIT
Select polyline or [Multiple]: l
Object selected is not a polyline
Do you want to turn it into one? <Y>Enter accepts. Measured on the object afterwards:
| Before, a spline | After, a polyline | |
|---|---|---|
| Vertices | 4 points passed through | 37 vertices |
| Area under the curve | 436,032.437 | 434,800.636 |
| Difference in area | — | 0.28% |
A smooth curve becomes 37 straight segments, 0.28% away from the original. For construction drawings that error is negligible. But two things are worth remembering. The change is one-way. And the smoothness follows the SPLINESEGS variable, 8 by default, so raise it before converting if you want a finer result. Polylines are covered in our article on polylines in AutoCAD.
PEDIT's Spline option does not give you a spline
This is the commonest confusion around the whole subject.
Take a polyline zigzagging through four points, run PEDIT and choose the Spline option. The name suggests a spline comes out, but measuring says otherwise:
| The original polyline | After the Spline option | |
|---|---|---|
| Object type | polyline | still a polyline, not a spline |
| Length | 2,422.282 | 1,663.481 |
| Area | 325,000 | 114,257.813 |
Two things at once. First, the object is still a polyline — a polyline drawn curved — so software that cannot read splines can still read it. Second, the curve cuts the corners hard: the length drops from 2,422 to 1,663 and the area to barely a third.
So that option is for softening a zigzag, not for converting to a spline, and it does not keep the original vertices. A line that must pass through the points needs the spline command in AutoCAD itself, in the fit method.
The distinction matters when a drawing comes back from a consultant and something that looks like a smooth curve refuses to behave like one. Select it and read the object type in the Properties palette: a genuine spline reports itself as a spline, while a softened polyline still reports a polyline. The two edit in completely different ways, and a good half of the frustration people have with curves in AutoCAD comes from working on one while thinking it is the other.
Drawing curves: which tool for what
Four ways to draw a curve, each with its place.
| Way | Passes through the points? | When to use it |
|---|---|---|
Arc, ARC | yes | A curve at a defined radius |
| Polyline with curved segments | yes | Mixed straight and curved, or a line width |
| Spline, fit method | yes | A smooth curve through many survey points |
| Spline, control vertices | no | A shape that has to look right and stay adjustable |
Arcs are covered in our article on drawing arcs in AutoCAD, and measuring these lines in our article on measuring areas in AutoCAD.
Four things people say that are not true
| What people say | What the 2027 release actually does |
|---|---|
| A spline always passes through the points you pick | Only the fit method does; control vertices touch nothing |
| The two methods differ only in how you click | The same four points give boxes of 1,011.46 and 510 |
PEDIT's Spline option converts a polyline to a spline | It stays a polyline, and the length drops from 2,422 to 1,663 |
| Converting a spline to a polyline ruins the shape | The area is 0.28% out, but it becomes 37 straight segments, one way only |
Frequently asked questions
What is the alias?
SPL. The ribbon button sits at Home → Draw → Spline CV.
How do I tell which method I am in?
Read the first line when the command starts: Current settings: Method=Fit or Method=CV. It is printed before the first prompt.
How do I switch methods?
At the first prompt type m, then fit or cv.
Why does my curve swing outside the points?
Because you are in the fit method. Four points spread 700 apart vertically give a bounding box of 1,011.46 — an overshoot of 311.46.
How do I turn a spline into a polyline?
Run PEDIT, pick the spline, and press Enter at Do you want to turn it into one?. Raise SPLINESEGS first for a finer result.
The other package will not open my spline — what now?
Convert it to a polyline before exporting, and keep a copy of the drawing with the spline intact so it can still be edited later.